We are using mutants of the yeast, Saccharomyces cerevisiae, to gain an understanding of the cell division process in eukaryotic cells. These mutants have identified 32 genes whose products play specific and necessary functions for the completion of cell division. We have devised a new procedure for determining the normal sequence of action of cell cycle mutants and inhibitors. Using this protocol we have determined the sequence with which several gene products act. Additional inhibitors are being sought that are cycle specific in order to continue this sequencing procedure further into the cell cycle. We are investigating the point in the cell cycle at which division is controlled. This point has been localized to the first step in the sequence of gene controlled steps and we are now devising procedures to select additional mutants that are deficient in completing this step. Mutants defective in DNA replication are being examined to see if they are thermolabile for any enzymes implicated in this process.